Such apparatuses are applied in a large number of industrial applications, especially in measuring- and control technology and in process automation, for determining and/or monitoring the named process variables.
In the case of the most well-known apparatuses of this type, the mechanically oscillatable structure has two oscillatory fork tines coupled via a membrane, which are caused to execute oscillations of opposite phase perpendicular to their longitudinal axis via an electromechanical transducer mounted on the rear membrane side, the side facing away from the tines. Along with that, there are also apparatuses known, whose oscillatable structure is only one oscillatory rod or an oscillatable membrane.
FIG. 1 shows a classical example of a corresponding apparatus, as such is applied for monitoring a certain fill level of a medium 1 in a container 3. The mechanical, oscillatable structure 5 includes here two oscillatory rods as tines coupled via a membrane and inserted laterally into the container 3 at the height of the fill level to be monitored. The structure 5 is caused to execute oscillations, for example, via an electromechanical transducer (not shown) arranged on the rear-side of the membrane. This is done, for example, by feeding the received, oscillation reflecting signal of the transducer back to the transducer via a phase shifter and an amplifier as excitation signal. For fill-level monitoring, for example, via the phase shifter, a fixed phase shift is predetermined, by which the resonance condition for the oscillatory circuit containing the mechanical oscillatory system as frequency determining element is defined. The frequency tuned by reason of the predetermined phase shift lies in the region of the resonant frequency of the structure 5, and is subsequently generally referred to as the oscillation frequency. The oscillation frequency is measured, and compared with an earlier determined switching frequency. If it is greater than the switching frequency, then the structure 5 is oscillating freely. If it lies below the switching frequency, then the structure is covered by the medium 1, and the apparatus reports an exceeding of the predetermined fill level.
Alternatively, in the case of a perpendicular insertion of a rod- or fork shaped, oscillatable structure into the medium, with corresponding calibrating based on oscillation frequency, the degree of covering and therewith the fill level over the length of the structure can be measured.
For determining and/or monitoring density or viscosity of the medium, the structure is inserted to a predetermined immersion depth perpendicularly into the medium, and the resulting oscillation frequency, or, in the case of an exciting with a fixed excitation frequency, the amplitude or the phase shift of the resulting oscillation is measured relative to the excitation signal.
An alternative form of excitation is frequency sweep excitation, in the case of which the frequency of the excitation signal passes periodically through a predetermined frequency range. Also, here, the process variable is determined and/or monitored based on the frequency of the resulting oscillation, the amplitude of the resulting oscillation, or the phase shift of the resulting oscillation relative to the excitation signal.
Today, always more frequently, apparatuses are being applied, which have only a single electromechanical transducer having at least one piezoelectric element, which is applied both for oscillation excitement as well as also for changing the resulting oscillation into an electrical, received signal. The received signal of the transducer corresponds, in such case, to a superpositioning of the excitation signal and a wanted signal representing the oscillation. These apparatuses have, compared with apparatuses with separate transmitting- and receiving transducers, the advantage that they are clearly smaller and more cost effective to manufacture.
The excitation signal is usually a rectangular, electrical, alternating voltage. This has the result that the piezoelectric capacitance of the transducer receives a reversal of the voltage sign at the edges of the rectangular signal, whereby charging- and discharging electrical currents arise, which are superimposed as disturbance signals on an electrical current representing the mechanical oscillation.
The charging- and discharging electrical currents of the transducer are suppressed, for example, by means of a compensation capacitor. Examples of this form of disturbance signal suppression are known from DE- 197 20519A1 and EP 1 750 104A2. The apparatuses described there have, in each case, a compensation capacitor supplied with the excitation signal in parallel with the transducer. Via the compensation capacitor, a reference signal is tapped, which is independent of the oscillation and corresponds to the excitation signal. Transducer and compensation capacitor have—in the case of equal capacitance—in reference to the reverse charging events, the same behavior. If the received signal tapped via the transducer and the reference signal tapped via the compensation capacitor are, as described in DE- 197 20 519A1, subtracted from one another, or, as described in EP 1 750 104A2, added after a preceding inverting, then the disturbance signals equally contained in the two signals cancel and there is provided on the output the wanted signal reflecting the mechanical oscillations of the structure.
While the capacitance of the compensation capacitors have no, or only a very low, temperature dependence, the capacitance of the transducer is, due to the high temperature dependence of the dielectric constant of piezoelectric materials, temperature dependent in high measure. This leads to the fact that the compensation of the disturbance signals functions yet poorer, the greater the temperature related difference between the capacitance of the transducer and the capacitance of the compensation capacitor.
This problem is solved in EP 1 750 104 A2 by matching the capacitance of the compensation capacitor provided for suppressing the reverse charging peaks by controlling the voltage across the compensation capacitor as a function of the capacitance of the transducer. For this, transducer and compensation capacitor are supplied supplementally with an auxiliary signal, whose frequency lies outside the frequency range, in which the resonant frequency of the mechanically oscillatable structure lies. In this frequency range, the structure executes a forced oscillation with the frequency of the excitation signal, so that received signal and reference signal contain, in the case of agreement of the capacitances of the two, exclusively the excitation signal and the disturbance signal. A control loop is provided, which controls the compensation voltage in such a manner that the reverse charging peaks of the reference signal compensate the reverse charging peaks of the received signal of the transducer in the case of the frequency of the auxiliary signal. In this way, the temperature dependent changes of the transducer capacitance are taken into consideration and cancelled in the compensation of the reverse charging peaks.
Temperature has, however, not only effects on the transducer capacitance, but, instead, acts especially also on the oscillation characteristics of the mechanical structure crucial for the determining and/or monitoring of the process variable. Thus, for example, the stiffness of the material of the oscillatable structure changes with temperature, which, in turn, means a temperature dependence of the resonant frequency, the oscillation amplitude, and the phase shift of the oscillation of the structure existing relative to the excitation signal.
In the case of monitoring a predetermined fill level, this means e.g. that the oscillation frequency occurring in the uncovered, respectively the covered, state of the oscillatable structure is subjected to temperature dependent fluctuations. Correspondingly, there is here, dependent on the temperature, the danger that a switching frequency, set earlier for the measured oscillation frequency for monitoring the exceeding, or subceeding, of the fill level, is not exceeded, or subceeded, in spite of an exceeding, or subceeding, of the predetermined fill level.
The temperature dependence of the oscillation characteristics leads to a marked temperature dependence of the accuracy of measurement of the apparatus.
In order, in spite of this, to assure an as exact as possible and reliable functioning of the apparatus, there is disclosed, for example, in DE 10 2006 007 199 A1 and WO 02/42724 A1, the idea of providing an additional temperature sensor in the region of the transducer, in order to compensate the influence of temperature, and to conduct, e.g. based on the therewith measured temperature, an adapting of the switching frequency.
An additional temperature sensor in the region of the transducer requires, however, space, is connected with additional costs, and must be connected electrically to the electronics of the apparatus, which is, as a rule, arranged removed from the transducer.